Label-free colorimetric and quantitative detection of cancer marker protein using noncrosslinking aggregation of Au/Ag nanoparticles induced by target-specific peptide probe

Assays for non-enzyme protein based on peptide–protein interaction are few due to the fact that most of peptide–protein bindings do not produce easily measurable output signals. Here we report a homogenous assay for colorimetric and quantitative detection of a cancer marker and promising antitumor target, cyclin A2, using noncrosslinking aggregation of unmodified AuNPs/AgNPs by utilizing the difference of coagulating ability of a cationic peptide probe (P1) and its binding form toward naked AuNPs/AgNPs. In the absence of cyclin A2, P1 coagulates particles immediately, whereas cyclin A2 binding prevents the interaction of P1 with metal particles surface, significantly reducing the magnitude of aggregation. The extent of aggregation is dependent on the concentration of the target protein cyclin A2 and the difference in color can readily be distinguished by spectrometer and naked eyes. The assay is sensitive and selective. Cyclin A2 assay using AuNPs as colorimetric indicator is more easily monitored by naked eyes owing to the distinct color change, and 40 nM cyclin A2 can be detected without the aid of any instruments. Using inexpensive desktop spectrometer, cyclin A2 assay using AgNPs as colorimetric indicator can detect as low as 30 nM cyclin A2, which is 20 fold lower than that of cyclin A2 assay using terbium-chelating peptide as the probe reported recently (Pazos et al., 2008, 130, 9652–9653). This strategy will shed light on developing of unlabeled peptide-based protein biosensors.